This invention relates generally to a process for the manufacture of hydrated calcium hypochlorite. More specifically it is directed to the product particle produced by the use of a turbine agglomerator in a process to produce a highly porous calcium hypochlorite product with a fast dissolving rate.
Calcium hypochlorite compositions have been commercially utilized for a number of years as bleaching and sanitizing agents in water bodies, particularly in the sanitation and disinfection of swimming pool waters. Calcium hypochlorite's commercial appeal lies in the fact that it is a relatively stable and inexpensive solid oxidizing agent that uses its available chlorine to remove impurities and kill pathogenic organisms in water.
Calcium hypochlorite has been manufactured or proposed for manufacture from lime and sodium hydroxide by a number of processes. The different processes are either directed to reducing waste materials from the process or to produce the best quality product that is contaminate free in the most cost effective manner.
In most of the proposed methods for commercially manufacturing calcium hypochlorite, a slurry is obtained containing crystals of calcium hypochlorite dihydrate in a concentrated aqueous solution of calcium hypochlorite and sodium chloride, or other inorganic halide. The slurry is then filtered to produce a cake containing from about 42 to about 48% by weight of water. When this cake is dried, a very light, porous cake is obtained which breaks down to an undesirable fine, dusty powder. The crystals in the cake lack natural cohesiveness. The filter cake can then be compressed, but the compressed cake, although harder, fragments into flaky granules with fragile edges. These granules are easily abraded and form an unsatisfactory dusty product. Alternately, the wet cake is partially dried., then compressed into a sheet between heavy rolls and further dried.
An alternate approach is the process of preparing calcium hypochlorite particles by admixing the wet cake of calcium hypochlorite in a cutting type mixer with dry fines in sufficient proportion to decrease the water content from the approximate 42 to about 48% by weight level down to a level of about 20 to about 30% by weight. Water is not evaporated during the mixing step, but rather the moist particles are dried in a separate step under carefully controlled conditions to avoid substantial crushing of the material. Compression pressures on the granules are less in this type of a mixer than with the aforementioned roll type of production. Therefore the mixer produced granules are softer. However, the granular particles so produced are not strong enough to resist dusting when subjected to severe handling conditions. In other techniques similar to this mixing technique, excessive dusting also has been a problem.
In all of the above-described calcium hypochlorite granulation techniques, drying is accomplished under generally gentle handling conditions in devices such as a belt dryer or a tray type of dryer. A belt or tray type of dryer is used to minimize dust formation and dust entrainment in the drying atmosphere. Thermal degradation is a problem in these types of dryers, however, because the drying rates are relatively slow and calcium hypochlorite is thermally sensitive.
An alternate approach has been developed through the use of a spray graining technique to produce product granules. Generally, the spray-graining technique employs the spraying and drying of calcium hypochlorite slurries to avoid filtration and drying problems. This technique also minimizes the loss of calcium hypochlorite due to thermal degradation by reducing the required drying time. A key consideration in this technique is the fact that calcium hypochlorite is susceptible to rapid chemical decomposition in the presence of moisture at temperatures only slightly above ambient room temperatures. The thermal stability of calcium hypochlorite improves as the water content is reduced. Spray graining techniques have been perfected to produce granular calcium hypochlorite particles with layers of calcium hypochlorite encapsulated in a smooth rounded granule. However, the apparatus necessary to produce these granules in a fluidized bed or rolling bed dryer, as well as the necessary careful control of conditions, have made this a costly technique. Also, the use of a rotary drum results in the buildup of moist materials on the inner drum walls that requires labor intensive scraping of the walls to remove.
Further, the techniques adopted to produce calcium hypochlorite have not necessarily led to the production of a durable granule of a particular size and shape that dissolves quickly in water. Dust produced from the recycling of particles during the manufacturing process normally must be compacted. These compacted particles are normally dense. These and other particles cannot dissolve completely when the granules are distributed on the surface of a swimming pool. Undissolved residue in pools is undesirable in calcium hypochlorite product, being aesthetically displeasing and possibly leading to the bleaching out of the color in pool liners when the residue settles on the bottoms of the pools.
Further, the highly corrosive nature of calcium hypochlorite takes its toll on the apparatus employed in its production. Where pressure or preform rollers and granulators are employed, costly regular replacement or repair of the equipment must be accomplished every year or two since rapid oxidation of the exposed metallic parts occurs. The preform rollers press the partially dried and filtered cake into thick sheets at high pressures, for example up to 1000 pounds per square inch. The pressed sheets are then fed into the granulators where the sheets are broken up into small particles by rotary blades forcing the product through screens of predetermined size to create more surface area for drying. These screens and blades are especially susceptible to corrosion.
Other costly handling equipment, such as elevators and conveyors, must be used to transport the filtered cake and crushed particles to, between, and away from the compressive and granulating apparatus.
These and other problems are solved in the application of the process to produce the agglomerated product particle comprising the present invention.